Radiation is converted to electron hole pairs in semiconductor material. In semiconductor radiation detectors the electron hole pairs are separated by an electric field. The charge type of the electron hole pair that is measured is referred to as signal charge and the opposite charge type is referred to as secondary charge.
Patent applications PCT/FI2004/000492 and PCT/FI2005/000359, which are incorporated herein by reference, disclose a semiconductor radiation detector having a modified internal gate (MIG). This detector is herein after referred to as the MIG detector. The MIG detector is back-illuminated and it has a thick fully depleted substrate and a thin conductive layer on the backside of the device. This conductive backside layer has three functions: when it is properly biased it enables full depletion of the thick substrate, it transports the secondary charges outside the active area of the device and it functions as a thin homogeneous radiation entry window. The MIG detector has many benefits. The surface generated charges can be separated from the signal charges, which provides a small dark current noise. The signal charge can be read non-destructively enabling the signal charge to be read multiple times which reduces the read noise. Back-illumination and the thin homogeneous radiation entry window enable the detection of shallow penetrating radiation like low energy X-rays and particles with a good energy resolution. The thick fully depleted substrate enables the detection of deeply penetrating radiation.
The substrate material of the MIG detector is preferably high resistive, i.e. almost intrinsic silicon and the thickness of the substrate is a few hundred micrometers. Such a MIG detector can be used for detecting particles, X-rays from low to medium energies (˜100 eV-˜10 keV) and photons from ultraviolet and blue light to near infrared radiation. Near infrared radiation is here referred to as radiation that can not be seen by the human eye and that has a wavelength below 1.1 μm which is the detection limit of silicon. Near infrared radiation which wavelength is close but below this limit has a very big attenuation depth in silicon, up to hundreds of micrometers. Due to back-illumination, due to the thick fully depleted substrate and due to the thin radiation entry window the MIG detector has high quantum efficiency from near infrared radiation to blue light. Due to the thick substrate also a phenomenon called fringing is eliminated. The fringing phenomenon is a problem in detectors having a thin substrate. In such detectors the near infrared radiation is reflected many times between the front and back surfaces of the detector before it is absorbed, causing unwanted interference patterns. Since the moonless night sky contains at least an order of magnitude more near infrared photons than visible photons and since the reflection coefficient of many materials is much higher for near infrared radiation than for visible light (for instance the reflectivity of foliage is three to six times higher) the MIG detector is very well applicable to low light detection in night vision devices.
The MIG detector does not, however, suite very well for the detection of visible light in silicon based portable consumer applications for the following reasons. The depletion of the thick substrate requires at least a few tens of volts. For a portable consumer device such a voltage is clearly too high and it results in too big power consumption. The high resistive silicon substrate is expensive and it is difficult to process, which increases the manufacturing cost. It is also difficult to contact the conductive back side layer reliably from the front side through the thick high resistive substrate, which would be important for mass production. A lot of bulk generation current is generated in the thick fully depleted substrate, which is likely to necessitate the use of cooling. In portable consumer applications the cooling of the detector is, however, not usually possible. The sharpness of the images is also somewhat degraded since visible light is absorbed on the back side of the detector and the signal charges have to drift a long way before they reach the front surface. For this reason also the use of colour filters on the back side of the device is problematic.
The attenuation depth of red light in silicon is not more than ten micrometers. For blue and green light the attenuation depth is even less. It is thus not necessary to have a thick substrate for visible light detection. Instead of the thick substrate one could use a thin (typically around 10 μm and less than 50 μm) substrate in a back thinned MIG detector. A thin device brakes, however, very easily and it is thus necessary to perform the back side processing at the end of the manufacturing process. There exist two possible methods to do this. In the first one the front side of the substrate is attached to a support substrate after which the back side of the detector is thinned. In the second one the back side of the detector is etched only below the active area containing the pixels and a thicker support area is left on the sides of the detector. In both of the methods it is required that the front side processing is finished before the back side is thinned. This fact complicates the manufacturing of the conducting back side layer. In order to process a very thin conductive back side layer enabling good quantum efficiency for blue light there exists two possible processes that are suitable for mass production. In the first method the conductive backside layer is done by implantation, which requires a high temperature annealing step. All materials that are used on the front side of the device, like the metal wirings, must have a higher melting point than the annealing temperature. This fact prohibits the use of many materials that are common in integrated circuits like aluminum. In the second method a thin layer is deposited on the back side of the device. A lot of dark current is, however, created at the interface between the conductive layer and the substrate and in order to suppress this current cooling is required.
There exists also an inherent problem related to the conductive back side layer in case the MIG detector is used for visible light detection. In order to detect badly illuminated areas of an image properly the size of the chip has to be large and a large optical aperture has to be used. In order to have also good quantum efficiency for the blue light the conductive back side layer has to be very thin. If the image contains also very bright areas, a lot of secondary charge current will be running in the conductive back side layer. The large current running in the conductive backside layer, and the small thickness and the large area of the conductive back side layer result in, however, a large resistive voltage drop in the conductive back side layer. This resistive voltage drop degrades the image quality and may lead to malfunction of the detector, especially if the detector is very thin.
Another problem in the MIG detector is that a relatively high voltage is required to clear the signal charge in the MIG especially if a high dynamic range is desired, i.e. if a large signal charge capacity of the MIG is desired. Yet another problem is that in some cases the isolation of the surface generated and signal charges should be improved in MIG detectors.